Conventionally, techniques for removing particulate matter (hereinafter referred to as “PM”) and nitrogen oxide (hereinafter referred to as “NOx”) contained in the exhaust gas exhausted from an internal combustion engine such as a diesel engine have been known.
Among the techniques, a selective catalytic reduction-type (SCR) exhaust gas purification apparatus is known as a technique for removing NOx. In the SCR-type exhaust gas purification apparatus, a reducing agent and an exhaust gas are mixed by injecting the reducing agent into an exhaust gas passage on the upstream side of an NOx catalyst. Then, the mixture of the reducing agent and the exhaust gas is passed through the NOx catalyst so that the NOx (such as NO and NO2) in the exhaust gas can be efficiently decomposed into nitrogen (N2) and the like, and the resulting exhaust gas is emitted.
One example of such a selective catalytic reduction-type exhaust gas purification apparatus is shown in FIG. 16. The exhaust gas purification apparatus 310 has an NOx catalyst 313 disposed in an exhaust gas passage 311, a reducing agent injecting device 315 for injecting a reducing agent into the exhaust gas passage 311 on the upstream side of the NOx catalyst 313, and upstream-side oxidation catalysts 317 and downstream-side oxidation catalysts 319.
In this exhaust gas purification apparatus 310, a urea aqueous solution injected into the exhaust gas as a reducing agent undergoes hydrolysis, whereby ammonia (NH3) is produced. The NH3 and the NOx react with each other in the NOx catalysts so that they are decomposed into nitrogen (N2) and (H2O), and the resulting exhaust gas is emitted to outside.
In such an exhaust gas purification apparatus, it has been demanded to diffuse the reducing agent efficiently in the exhaust gas that flows into the NOx catalyst, and for that purpose, an exhaust gas purification apparatus in which a perforated plate is disposed at the entrance of the NOx catalyst has been proposed. More specifically, an exhaust gas purification apparatus as shown in FIG. 17 is disclosed (see Patent Document 1). In the exhaust gas purification apparatus, a DPF 333 and a urea NOx removal catalyst 323 are disposed in parallel (the flow of the exhaust gas is in-line). These are allowed to communicate with each other by a communication chamber 330, and the entire apparatus is formed in an angular U-shape. Also, a perforated plate 338 in which a large number of holes 338A are drilled is disposed at a mid position along the exhaust gas flow direction in an upstream-side communication chamber 330 of the urea NOx removal catalyst 323.    Patent Document 1: JP-A-2005-155404 (Paragraphs [0038] to [0039], FIG. 1)